This invention relates to a method of destroying a waste gas comprising ammonia.
Waste gas streams comprising ammonia are frequently encountered in refineries. Sometimes such waste gas streams also contain hydrogen sulphide in comparable proportions. These gas streams can be employed as a feed stream to the Claus process.
Other waste gas streams formed in refineries can contain little or no hydrogen sulphide but may contain ammonia, sometimes as essentially the only combustible component thereof. There is a need to destroy essentially all of the ammonia in such gas streams but without creating appreciable amounts of oxides of nitrogen in the effluent gas arising from the destruction process.
It is an aim of the present invention to provide a method of destroying a waste gas stream containing at least 50 per cent by volume of ammonia but essentially no hydrogen sulphide which makes it possible to solve the above problem.
We have surprisingly discovered that essentially pure ammonia streams can be successfully destroyed without creating appreciable quantities of oxides of nitrogen by employing oxygen-enriched air or its equivalent to support combustion of the ammonia, provided that sub-stoichiometric combustion conditions are maintained in the furnace in which the destruction is carried out.
According to the present invention there is provided a method of destroying a waste gas containing at least 50 per cent by volume of ammonia, comprising the steps of:
(a) supplying at least one stream of the waste gas to a reaction region;
(b) supplying molecules of oxygen to the reaction region either in at least one stream of oxygen-enriched air or in separate streams of (i) air unenriched in oxygen and (ii) pure oxygen or oxygen-enriched air;
(c) both burning and thermally cracking ammonia in the reaction region; and
(d) taking from the reaction region an effluent gas stream, comprising nitrogen, water vapour, argon, and hydrogen but being essentially free of nitric oxide, nitrogen dioxide, and dinitrogen tetroxide and essentially free of ammonia, wherein
(i) the mole ratio of oxygen molecules to all non-combustible gas molecules (including oxygen molecules) supplied to the reaction region is in the range of 28:100 to 70:100;
(ii) the rate of supplying oxygen molecules to the reaction region is from 75 to 98% of the stoichometric rate required for full combustion of all combustible fluids supplied to the reaction region;
(iii) and essentially no hydrogen sulphide is supplied to the reaction region.
By employing oxygen-enriched air, or equivalent separate supplies of oxygen and air, containing at least 28 mole per cent of oxygen, it becomes possible to create in the ammonia flame a sufficiently high temperature to cause some of the ammonia to crack thermally to form ammonia and hydrogen. This sub-stoichometric oxidation conditions can be maintained in the reaction region while obtaining essentially complete destruction of ammonia. A consequence of operation under sub-stoichometric oxidation conditions is that any oxides of nitrogen that are formed, i.e. nitric oxide, nitrogen dioxide or dinitrogen tetroxide, can be reduced to nitrogen by reductant(s) present in the reaction region.
Preferably, the rate of supplying oxygen molecules to the reaction region is from 80 to 90 per cent of the stoichometric rate required for full combustion of all combustible fluids supplied to the reaction region. At such a rate a favourable combination of destruction of ammonia by combustion and by thermal cracking can be achieved.
Preferably, the reaction region, or at least part of it is defined in a furnace. Preferably, the flow rate of at least one influent stream into the furnace is controlled so as to maintain the temperature of the effluent gas at the exit of the furnace in the range of 1300xc2x0 C. to 1700xc2x0 C. At temperatures above 1700xc2x0 C., damage tends to be done to the furnace, particularly any refractory lining thereof.
Preferably, the effluent gas contains more than 4 per cent by volume of hydrogen. More preferably, the effluent gas contains from 5 to 10 per cent by volume of hydrogen. Such effluent gas compositions are typically flammable and can therefore be burned to form a tail gas which may be discharged to the atmosphere, if necessary, after removal of any remaining traces of ammonia by dissolving such traces in water or other aqueous medium.
Selecting a mole ratio of oxygen molecules to all non-combustible gas molecules (including oxygen molecules) supplied to the reaction which is significantly above the minimum of 28:100, and a rate of supplying oxygen molecules to the reaction region of less than 90 per cent of the stoichometric rate required for full combustion of all combustible fluids both facilitate the production of an effluent gas stream which can readily be burned.
If, for example, the effluent gas stream is not flammable or is not able to sustain a stable flame, the concentration of hydrogen in it can be enhanced by any of the following measures if taken upstream of the combustion of the effluent gas stream:
(i) condensation or adsorption of at least part of its water vapour content;
(ii) separation by PSA or membranes to enhance its hydrogen content;
(iii) addition of a fuel gas thereto.
Measure (i) may be performed by cooling the effluent gas stream and contacting the cooled effluent gas stream with water or other aqueous medium.
Although it is generally performed to burn the effluent gas stream and discharge the resulting tail gas to the atmosphere, if desired, after treatment of the effluent gas stream to remove the last traces of ammonia therefrom, other methods of treating the effluent gas stream are possible. For example, the effluent gas stream may be subjected to separation so as to obtain a more concentrated fuel gas such as essentially pure hydrogen product. Another option, which is preferred if a small amount of hydrogen sulphide is supplied to the reaction region, is to supply the effluent gas stream to a unit for cleaning a tail gas from a Claus plant.
The expression xe2x80x9cessentially no hydrogen sulphide is supplied to the reaction regionxe2x80x9d should be understood to encompass the supply of hydrogen sulphide to the reaction region at a low rate, i.e. such that up to 5 mole per cent of the combustibles supplied to this region is formed of hydrogen sulphides with the effluent gas stream being suitable for treatment in a unit for cleaning a tail gas from a Claus plant.
The reaction region is readily operable so as to avoid the creation of an effluent gas stream that contains any of ammonia, nitric oxide, dinitrogen tetroxide, and nitrogen dioxide.
Preferably, all the waste gas is fed to a burner which fires into the reaction region. Alternatively, some of the waste gas may be introduced into the reaction region downstream of the flame created by operation of the burner. The ammonia introduced into the downstream region can then react with any nitric oxide, nitrogen dioxide or dinitrogen tetroxide in the combustion products produced by operation of the burner.
The effluent gas stream is preferably burned in a further furnace into which a further burner fires. The entire effluent gas stream is preferably supplied to the further burner. Combustion of the effluent gas stream may be supported by air or pure oxygen, or by oxygen-enriched air. The effluent gas stream is preferably cooled intermediate the furnaces.